Why do aliens always have to look so much like humans? Really, what are the chances of aliens having two arms, two legs, eyes, a nose and a mouth.
And just because plants on Earth use chlorophyll, doesn't mean photosynthesising alien life will contain chlorophyll. There's loads of other pigments, and there would probably be many more than haven't evolved on this planet

Micky T.
There are some principles that are universally valid in evolution, no matter where you live.
Generally, it is better to have all your "thinking structures" in one place - in humans, inside the head.(the tendancy of centralisation). Also it is pretty probable that all that "thinking structure" will be inside the head of aliens, as that is the part that first meets the environment and it is logical to have most of your sensors there. also, two eyes are the most probable solution, as they give the highest coverage/energy expeditures ratio. and so on, so on. it doesn't mean that martians would resemble humans as much as hollywood pictures say they do, but my guess is that they would have some resemblances, affirmation I base on the principles of convergent evolution and Occam's raisor.

About clorophil. First of all, it is highly unlikely that a moving creature would photosynthesise, cause that would mean it would have to be extremely inactive(photosynthesis gives off very few energy). And would they be green? Well, i don't know. you would have to check with an astronomer about the weather conditions of Mars. They won't be red, i can tell you that, because they need to absorb red light which is the only one present in the morning and evening, even on Mars. I don't know if they would be blue though. Plants on earth are not blue cause they need blue light when it is cloudy outside. does mars have clouds? and can blue light pass through them? if the answer is yes, then yes, probably any ipothetical photosynthetic organisms on mars would use a green pigment like clorophyl...

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

Surely for convergent evolution to occur, the two organisms must be in the same, or at least similar environments. If we take Mars for example - the environment is completely different.

If we ever find aliens, I believe we may not even know if they are alive or not. They won't necessarily show the same evolutionary mechanics then those that occured on Earth - evolution might not even exist for them. They might not contain DNA. We can't really tell until we see one, which is quite improbable, due to the large distances between stars, and even larger distances between galaxies

there are some principles that are universal. it makes sense to have your receptors in the front part, because that is the part that meets the environment first. centralisation is also very probable, as shorter distances allow better communication.

It is easy to say "conditions are different, so they will be totally different" but it is not quite scientific. a scientific opinion is trying to understand why something happens, and see if that "why" would also be present there..

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

I'm only saying how we can possibly think of an alien without even seeing one. If aliens did arrive on Earth, and they looked human, or have human like features, then we could look at it scientifically. As it is, it's all guesswork.

We can't really guess due exclusively to what we have seen on Earth, as this is only one planet.

But it's still quite a pointless idea, seeing as we are probably never going to see aliens